I know this thread has been defunct for a while, but I’ve struggled with this early on and want to add my thoughts here…

Everything mentioned above is good advice - but I've had similar issues with curling/lifting off of my prints, especially at the corners. What I've seen is the more surface area a print's first layer has, the more likely one or more corners will lift. A great way I've found to mitigate this involves adding some brim-type structures to the model before printing. I call them "Mickey Mouse Ears" (MME’s), and they work extremely well at keeping the base layer attached to the bed. The downside to this fix is that you'll have to use some kind of design software to open the .stl or model and add these structures. I have found that I can make nice flat/non-curled prints without using a heated bed, glass plate or having to use acetone/gluesticks/hairspray...and I don't have to futz around with extruder temps. BTW, I am doing all this on my [highly-Borgified] Printrbot Simple, although I have a Makerfarm 12" i3v coming any day now. I print everything at 200 C and it has worked very well for me

I use PTC Creo2 for most of my modeling needs, especially to create parts/ assemblies and add custom support structures to my downloaded [SciFi] models to make them print properly. I use this software in my profession and have my own copy of it at home so it works well for my situation. I think there are some freeware/inexpensive design software packages that might be helpful for doing this, perhaps others on this forum can supply info on what works for them.

Also some specifics: my Simple has the extended aluminum bed with painter’s tape (no other surface prep). I have only been printing in PLA as I don’t have a heated bed (looking forward to the i3v so I can try other materials!). I make the MME’s pretty thin, just the first two layers and this seems to work very well. I usually print at 200 microns so when I model the MME’s I make them .4mm thick in Creo2, that way I get the two-layer result I want. The only other downside is that it takes a little added effort to remove them, but since they are rather thin I just use an X-acto knife. To me the slight additional work is well worth having perfectly flat parts!

If you don't want to/can't work with a software design tool then adding a brim might be the next best thing, in addition to all the great ideas shared above.

Just wanted to share what has worked well for me – hope this helps anyone who might be having similar issues with parts warping. I've included a couple pics below showing the "Mickey Mouse Ears" that I added to a rather complicated part. It came out perfectly flat and the MME's were pretty easy to remove.

MME example1.jpgMME example2.jpg